1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus having a preregistration mechanism.
2. Description of the Related Art
In image forming apparatuses, it is desired to convey paper at the shortest possible intervals to achieve high productivity.
When paper conveyance intervals are shortened, it is necessary to accurately align the timings at which paper is conveyed. On the other hand, in a paper conveyance unit, the paper conveyance speed may vary with variations in the paper initial position, variations in the diameter of the conveyance roller, and variations in the surface state of the conveyance path. Various techniques for adjusting those variations have been disclosed.
For example, Document 1 (Japanese Laid-Open Patent Publication No. 2002-029649) discloses a technique for adjusting variations as described above using preregistration correction. Preregistration correction is a process of adjusting a timing at which paper is conveyed to a registration position by temporarily stopping paper upstream from the registration position. Preregistration correction will be described with reference to FIG. 13.
FIG. 13 is an operation diagram showing operation paths of the front and back ends of paper in a conveyance path. In the example shown in FIG. 13, on the conveyance path, a paper feed roller (paper feed R) picking up paper from a paper feed cassette, a paper feed sensor (paper feed S) detecting the fed paper, a conveyance sensor (conveyance S) detecting passage of paper on the conveyance path, a registration sensor (registration S) detecting paper at a registration position, a registration roller (registration R) passing paper, stopping at the registration position, to a prescribed position such as an image forming unit are arranged in this order.
In FIG. 13, line L9A shows a front end position of the first sheet of paper conveyed through the conveyance path, line L9B shows a back end position of the first sheet of paper, line L9C shows a front end position of the second sheet of paper, and line L9D shows a back end position of the second sheet of paper.
When paper is conveyed from the paper feed cassette, the conveyance speed may vary among sheets of paper depending on, for example, the arrangement of paper in the tray or the slip amount of the conveyance roller. In FIG. 13, a variation in the conveyance speed between sheets of paper conveyed from the paper feed cassette is shown by line L91 and line L92. FIG. 13 shows a change of the front position when paper conveyance is stopped as preregistration correction based on that the paper feed sensor detects paper. When the first sheet of paper starts being fed at time T91, paper shown by line L91 arrives at a stop position for preregistration correction (preregistration position) at time T92, and paper shown by line L92 arrives at the preregistration position at time T93. When it is assumed that the arrival time varies in this way, the conveyance from the stop position should be resumed later than time T93 (time T94). In other words, when it is assumed that the time at which paper arrives at the stop position varies, it is necessary that the conveyance should be resumed later than the latest possible time.
When the registration sensor detects the paper after the conveyance of the first sheet of paper is resumed at time T94, the conveyance of the paper is stopped again so that the paper stops before the registration roller (the registration position). A variation in the front end position of paper after resumption of conveyance is shown by lines L93 and L94. The times at which paper shown by lines L93 and L94 reach the registration position are shown by times T95 and T96. The variation in arrival time at the registration position (the interval between time T95 and time T96) is smaller than the variation in arrival time at the preregistration position (the interval between time T92 and time T93). In this manner, preregistration correction can reduce variations of the arrival time at the registration position.
When the paper feed sensor detects the back end of the first sheet, the conveyance of the second sheet is started. The variation in the front position of the second sheet is shown by lines L95 and L96.
Document 2 (Japanese Laid-Open Patent Publication No. 2009-249093) discloses a technique including a sensor for detecting paper on a conveyance path, in which a conveyance speed for each sheet is adjusted by adjusting the conveyance speed based on the time from paper feeding to detection of paper by the sensor.
According to Document 1, although the timing at which paper is conveyed to the registration position can be adjusted, the interval from the preceding paper (the interval between the first sheet and the second sheet) increases. In order to shorten this interval, the distance from the preregistration position to the registration position may be increased, and the conveyance speed from the preregistration position to the registration position may be made higher than the conveyance speed downstream from the registration position. However, if the distance from the preregistration position to the registration position is longer, the variations in the paper arrival timing at the registration position increase, and therefore, the timing at which the conveyance is resumed at the registration position has to be delayed. Accordingly, the interval from the preceding paper increases downstream from the registration position.
In the technique described in Document 1, it is preferable to bring the preregistration position closer to the registration position in order to reduce variations in the arrival timing at the registration position, while it is preferable to increase the distance from the preregistration position to the registration position in order to shorten the interval between sheets. Thus, such contradictory conditions should be considered.
Although the technique described in Document 2 may contribute to higher productivity without stopping paper, it may raise a new problem of variations in conveyance speed, for example, due to accuracy of measurement of the sensor.